1. Field
Example embodiments generally relate to cooling systems for spent nuclear fuel (also known as “used nuclear fuel”). Example embodiments also generally relate to casks including cooling systems for spent nuclear fuel (“SNF”). Additionally, example embodiments generally relate to methods for cooling SNF.
2. Description of Related Art
Nuclear fuel discharged from the reactor of a nuclear plant is known as SNF. SNF is intensely radioactive. The associated radioactive decay creates heat, requiring some mechanism for cooling the SNF. Typically, SNF is initially stored in pools of water or other coolant (known as “wet storage”). The water or other coolant may provide both radiation shielding and cooling.
As SNF ages, the radioactivity level may drop, as may the associated heat generation. At some point, the heat generation may drop to a point at which wet storage is no longer required. After meeting the minimum wet storage period, the SNF may be removed from the pool and placed in appropriate dry transportation and/or storage systems (known as “dry storage”).
SNF from light water reactors is typically cooled at least 5 years in wet storage. Although the Nuclear Regulatory Commission (“NRC”) has authorized transfer to dry storage as early as 3 years, the industry norm is about 10 years.
In contrast, SNF from liquid-metal-cooled reactors may be, for example, stored within the reactor vessel pool for a fuel cycle (e.g., 18-24 months) before transfer to dry storage.
In the U.S., two basic types of dry transportation and/or storage systems are used, bare-fuel casks and canister-based systems.
In bare-fuel casks, assemblies of SNF typically are placed into a basket that is integrated into a cask and the cask is then sealed. In canister-based systems, assemblies of SNF typically are placed into baskets integrated into a thin-walled cylinder, referred to as a canister, and the canister is then sealed. For both types of casks, transportation and long-term storage typically require the use of an overpack to protect the cask or canister against external man-made events and external natural phenomena.
The stages of the nuclear fuel cycle may be considered to include processing, enrichment, fabrication of the nuclear fuel, use of the nuclear fuel, storage of the SNF, and reprocessing of the SNF for the enrichment stage. Due to well-publicized problems associated with the reprocessing stage, the storage stage for SNF has become much more important. And due to well-publicized problems associated with the storage stage, particularly concerns about wet storage, the ability to move SNF from wet storage to dry storage as quickly as possible and to store the SNF in dry storage for as long as possible have become the focus of considerable research. Both the ability to move SNF from wet storage to dry storage and to store the SNF in dry storage depend on the ability of the associated dry transportation and/or storage system to dissipate the heat created by the radioactive decay of the SNF.